The present invention relates to photographic color printing apparatus in general, and more particularly to improvements in exposure control units for photographic color printing apparatus. Still more particularly, the invention relates to improvements in exposure controls of the type which comprise means for scanning and analyzing different areas or fields of originals to be copied.
German Offenlegungsschrift No. 1,597,066 discloses a color printing apparatus wherein the original is assumed to be subdivided into several fields each of which is monitored in order to determine the average contrast. The exposure is controlled in dependency on the results of scanning of that field which exhibits the maximum average contrast. The operation of such printing apparatus is based on the premise that the important part of an original (i.e., that part which should be reproduced with a maximum degree of accuracy) normally exhibits a high average contrast. This insures that the exposure controls do not take into consideration relatively large areas of identical color, i.e., those areas of the scene which exhibit so-called subject failure, such as would cause a shift in the color balance when the operation of the apparatus is based on the so-called neutral grey compensation principle.
A drawback of the just described printing apparatus is that the determination of average contrast is a complex procedure and necessitates resort to expensive and highly sensitive equipment. Moreover, only a relatively small portion or field of the original is taken into consideration for determination of the quantities of printing light for the entire reproduction. This follows necessarily because the monitoring of a major area could embrace the scanning of a relatively large area of predominant color which would produce subject failure.